In manufacturing surface-mount printed circuit boards, a stencil printer can be used to print solder paste onto the circuit board. Typically, a circuit board having a pattern of pads or some other, usually conductive, surface onto which solder paste will be deposited is automatically fed into the stencil printer; and one or more small holes or marks (known as xe2x80x9cfiducialsxe2x80x9d) on the circuit board are used to properly align the circuit board with the stencil or screen of the stencil printer prior to printing solder paste onto the circuit board. In some systems, an optical alignment system is used to align the circuit board with the stencil.
Once the circuit board has been properly aligned with the stencil in the printer, the circuit board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures in the stencil and onto the board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain a desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste typically is dispensed onto the stencil from a standard cartridge such as that manufactured by Systems Engineering and Management Co. (SEMCO), Vista, Calif., USA.
In some stencil printers, any excess solder paste remaining under the squeegee, after it has fully traversed the stencil, remains on the stencil when the squeegee is returned to its initial position for printing on a second circuit board. In some screen printers, a second squeegee moves across the stencil in the direction opposite to that of the first squeegee. The first squeegee and the second squeegee are used on alternating boards to continually pass the roll of solder paste over the apertures of a stencil to print on each successive circuit board. In the stencil printers that utilize two squeegees, there is still the problem that at the end of a manufacturing day, or when the stencil is to be changed, excess solder paste typically remains on the stencil and must be manually removed. Also, in these known printers, it is difficult to maintain a desirable viscosity because volatile solvents escape from the solder paste, thereby affecting the viscosity of the solder paste.
In the stencil printers discussed above, the squeegee blades are typically at a predetermined angle with respect to the stencil to apply downward pressure on the solder paste to force the solder paste through the apertures in the stencil as the squeegee is moved across the stencil. The angle of the blade is selected based on the speed at which the blade traverses the stencil and based on the desired downward pressure on the solder paste from the blade. It is desirable to maintain a consistent pressure on the solder paste as the squeegee traverses the stencil; however, in a typical printer, the pressure varies due to variations in paste viscosity throughout a production run and due to variations in the angle of the squeegee caused by deformation of the squeegee due to the pressure applied by the squeegee driving device.
Responding to some of the problems, noted above, in previous printing apparatus, an improved solder-paste dispensing head is described in U.S. Pat. No. 5,947,022, wherein a movable dispensing head has a cylindrical chamber including ports to which removable cartridges that supply solder paste are coupled. Solder paste is passed from the removable cartridges, into the cylindrical chamber, then out of a dispensing slot, through a stencil and onto a circuit board in a desired pattern. U.S. Pat. No. 5,947,022 is incorporated herein by reference in its entirety.
Nevertheless, even in this improved solder-paste dispensing system, consistency in the quality and character of the printed solder paste can not always be maintained for all types of solder paste, and consistency in the quality of the finished printed circuit board consequently suffers. Further, the stencil printing process generally is still not understood well enough that it can be reliably engineered to regularly produce acceptable levels of defects in the printed solder.
The printer of this invention includes a stirring mechanism that can offer a substantial improvement in, and control over, the quality of printed material. The stirring mechanism can be used to preserve and promote the homogeneity of the printed material. When the printer is used for printing solder paste, the stirring mechanism can prevent or reduce compaction of the solder paste during the printing operation. When solder paste xe2x80x9ccompacts,xe2x80x9d the flux separates from the solid particles, thereby compromising the quality of the printed solder paste.
More specifically, the printer of this disclosure includes a viscous-material dispensing head that has a chamber through which the viscous material (e.g., solder paste) can be channeled. The chamber includes at least one source port that can be coupled with a source of viscous material, such as a solder-paste cartridge, and a dispensing slot through which the viscous material can exit the chamber and be deposited on a substrate. In operation, the viscous material is directed from the source through the source port and into the chamber; the viscous material then exits the chamber through the dispensing slot.
The stirring mechanism of this invention is mounted within the chamber and is coupled with a drive mechanism that displaces the stirring mechanism through the chamber so as to stir the viscous material within the chamber. In embodiments where the viscous material is solder paste, displacement of the stirring mechanism through the chamber stirs the solder paste to greatly reduce or eliminate compaction of the solder paste, which would otherwise occur.
In one embodiment, the drive mechanism reciprocally displaces the stirring mechanism along a longitudinal axis of the chamber. The chamber can be substantially-cylindrical in shape, with the source port(s) and dispensing slot(s) radially displaced from the longitudinal axis on the side wall of the chamber. In a particular embodiment, the stirring mechanism is an apertured stirring blade mounted on a threaded shaft (i.e., a screw), wherein the stirring blade is displaced along the threaded shaft as the shaft is rotated by a rotary motor or other drive mechanism. As the stirring blade is displaced, viscous material (e.g., solder paste) is displaced through the apertures of the blade and thereby stirred.
The dispensing head can be mounted to a frame, to which a stencil is also mounted. The stencil is positioned between the dispensing slot of the dispensing head and the substrate (e.g., a printed circuit board) on which solder is to be deposited at selected locations.
Another advantage that can be achieved with apparatus and methods of this invention is that the printers can be used to print a broader variety of viscous materials having a broader range of viscosities because the speed of the stirring mechanism can be adjusted to provide as much or as little stirring as is necessary to preserve the homogeneity of a particular viscous material.